2014年GRE考试阅读模拟试题5-人人终身学习知识网~是各类综合知识资源信息分享，提升综合素质与提高知识技能的终身学习网络平台GRE留学英语

meili 发表于 2022-10-18 19:13:29

2014年GRE考试阅读模拟试题5

Many theories have been formulated to explain therole of grazers such as zooplankton in controlling theamount of planktonic algae in lakes.The first theories of such grazer control were merely5 based on observations of negative correlationsbetween algal and zooplankton numbers. A low numberof algal cells in the presence of a high number ofgrazers suggested, but did not prove, that the grazershad removed most of the algae. The converse obser-10 vation, of the absence of grazers in areas of highphytoplankton concentration, led Hardy to proposehis principle of animal exclusion, which hypothe-sized that phytoplankton produced a repellent thatexcluded grazers from regions of high phytoplankton15 concentration. This was the first suggestion of algaldefenses against grazing. Perhaps the fact that many of these first studiesconsidered only algae of a size that could be collectedin a net , a practice that over-20 looked the smaller phytoplankton that we now know grazers are most likely to feed on,led to a de-emphasis of the role of grazers in subse-quent research. Increasingly, as in the individualstudies of Lund, Round, and Reynolds, researchers25 began to stress the importance of environmentalfactors such as temperature, light, and water move-ments in controlling algal numbers. These environ-mental factors were amenable to field monitoring andto simulation in the laboratory. Grazing was believed30 to have some effect on algal numbers, especially afterphytoplankton growth rates declined at the end ofbloom periods, but grazing was considered a minorcomponent of models that predicted algal populationdynamics.35 The potential magnitude of grazing pressure onfreshwater phytoplankton has only recently beendetermined empirically. Studies by Hargrave andGeen estimated natural community grazing rates bymeasuring feeding rates of individual zooplankton40 species in the laboratory and then computing com-munity grazing rates for field conditions using theknown population density of grazers. The high esti-mates of grazing pressure postulated by theseresearchers were not fully accepted, however, until the45 grazing rates of zooplankton were determined directlyin the field, by means of new experimental techniques.Using a specially prepared feeding chamber, Haneywas able to record zooplankton grazing rates in naturalfield conditions. In the periods of peak zooplankton50 abundance, that is, in the late spring and in the summer,Haney recorded maximum daily community grazing rates,for nutrient-poor lakes and bog lakes, respectively, of 6.6percent and 114 percent of daily phytoplankton production.Cladocerans had higher grazing rates than55 copepods, usually accounting for 80 percent of thecommunity grazing rate. These rates varied seasonally,reaching the lowest point in the winter and early spring.Haneys thorough research provides convincing fieldevidence that grazers can exert significant pressure on60 phytoplankton population.

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人人终身学习知识网~是各类综合知识资源信息分享，提升综合素质与提高知识技能的终身学习网络平台's Archiver

2014年GRE考试阅读模拟试题5